The present invention relates to a safety method and device for full-voltage power supply to a user apparatus only when required for operation of same.
In particular, the user apparatus is an actuator unit of the type used for moving beds, arm-chairs and the like.
In the known art the necessity to avoid a user apparatus being connected to the mains when not in use has been felt, in order to eliminate the risk of accidental electrocution and reduce electric consumption to no purpose.
In the known art safety devices have been proposed which comprise a relay that under rest conditions causes insulation of the user apparatus from the mains. An appropriate circuit powers the relay when a suitable command in the user apparatus is operated. For example, this command may be accomplished by suitable connection of a control element already present in the user apparatus for controlling some proper functions of the latter. In order to be able to have the required electric power for activating the relay when the circuit is not connected to the mains, use of an electric battery or a great-capacitance capacitor that are charged during operation of the user apparatus has been proposed. EP-A-0 615 667 discloses an example of a safety device of this type.
Such a device however suffers from the drawbacks of being adapted for use only with an expressly designed user apparatus and of depending on a battery or a storage capacitor; as a result, there is an important cost and the risk that, due either to a long period of inactivity bringing about an inevitable discharge of the battery or capacitor or to the battery failure, the user apparatus cannot start operating when required. Furthermore, in addition to the normal power-supply connection, an additional electric connection between the safety device and the user apparatus is to be provided.
Safety devices powering the user apparatus through high impedances when normal operation is required have been also proposed. An increase in the voltage drop on the impedance indicates a current requirement by the user apparatus and causes operation of a relay short-circuiting the impedance and enabling full-power supply to the user apparatus. Such devices however cannot be employed in a satisfactory manner in the case of low-absorption user apparatuses or apparatuses having small absorption variations when powered to a sufficiently low voltage so as to eliminate electrocution risks. This, for example, occurs in the case of user apparatuses such as movement actuators for beds, arm-chairs or the like, in which the circuit portions of relatively high absorption are powered through relays that do not operate at much lower voltages than the rated voltages, and/or in which power supply takes place by means of step-down transformers and when the operating commands are disposed downstream of said transformers. In fact, in these cases, absorption variations at the user apparatus inlet, when said apparatuses are powered through sufficiently high impedances so that a satisfactory protection against electrocution is provided, become too much reduced and often uncertain and masked by normal noise present along the electric lines.
It is a general object of the present invention to obviate the above mentioned drawbacks by providing a safety method and device capable of enabling full power supply of a user apparatus only when needed, without requiring additional connections and modifications to the user apparatus and ensuring a correct operation even with user apparatuses powered through feeder transformers, and/or with small absorption variations when powered at sufficiently low voltages to ensure the absence of electrocution risks.